1. Field of the Invention
This invention relates generally to valves and actuators therefor and, more specifically, to a Pendulum-type Valve having Independently and Rapidly Controllable Theta- and Z-axis Motion.
2. Description of Related Art
Pendulum valves, also known as gate or slide valves are particularly suited for systems mandating large diameter flow conduits such as semiconductor manufacturing, thin film and vacuum process equipment. Specifically, in many such processes, the process chamber is placed under a vacuum condition prior to, during and after engaging in whatever process is conducted in the chamber. This typically involves the metered introduction of small amounts of certain gases into the rarified internal atmosphere (of the chamber) to achieve the required chemistry and pressure conditions within the chamber. In any such process, the ability to rapidly evacuate (empty) the process chamber of gaseous molecules such as process byproducts and other chemicals, is of critical importance. In order to achieve such precise control and rapid evacuations, a high-throughput-volume vacuum pump is connected to the exhaust of the process chamber by large diameter piping; the vacuum pump is “connected” and “disconnected” from the process chamber by a valve capable of opening as wide as the piping bore (to eliminate any flow restrictions), and then close very tightly to precisely increase or maintain desired pressure, and to completely isolate the vacuum pump from the chamber.
The pendulum valve has historically been the valve best suited for isolating the vacuum pump from the process chamber because it can open wide until the valve plate is completely out of the process flow path (to allow for unrestricted flow and maximum conductance), and can then be closed and sealed tightly to achieve a secure and complete isolation between the vacuum pump and the vacuum chamber.
But as critical as a fully opening and closing (sealing) valve is to the aforementioned vacuum processes, of even higher importance is the ability to precisely control (throttle) the vacuum level (pressure) in the vacuum chamber within certain desired parameters. These parameters primarily include time, accuracy, stability, and flow symmetry in the vacuum chamber, all of which are strongly influenced by the actuation ability and flow symmetry achieved through the pendulum valve.
In order to clarify valve plate positioning for later reference herein, the valve plate, while having a multitude of optional positional locations within the valve housing, can be described as having three cardinal locations: a first open position where the valve plate is completely removed from the flow path through the valve housing, and the plate has moved as far away from the valve seat in the Z-axis direction as it can; a second open position where the valve plate is completely eclipsing the flow path, with the valve plate remaining at maximum z-axis stroke away from the valve seat in the Z-axis direction as it can; and a third closed position where the valve plate is completely eclipsing the flow path and the valve plate is being pressed against the valve seat in the Z-axis direction with all available sealing force. Moving from the first open position to the second open position involves movement of the valve plate solely in the theta direction, while moving from the second open position to the third closed position involves movement of the valve plate solely in the Z-direction.
There are several drawbacks inherent to the conventional “throttling” or control pendulum valve design and actuator mechanism, several of which make meeting all control and sealing parameters particularly challenging. In the conventional pendulum valve, there are essentially two discrete valve positions—full open and full closed (sealed). In addition, a multitude of intermediate positions can be effected by using a variable position valve actuator, such as a motor, which can position the valve plate in positions between full open and full closed so as to achieve the desired flow throttling. In such a manner, the valve plate swings open and closed in what is sometimes referred to in the “theta” direction. Once the valve plate is fully covering the flow path, it then moves in the “z” direction, which is a direction in line with the flow path, until the valve plate seals against the valve housing. It is in this small axial motion that the majority of the process control at low absolute pressure (high vacuum) and low flow of metered gases occurs.
Conventionally, there can be no z-direction control of the plate until the theta direction of motion has distinctly terminated with the valve plate in exactly the near-closed (theta) position, because there needs to be enough of a gap between the valve plate and the valve housing to allow for the plate to swing freely through the entire theta path. Since the two motions cannot conventionally be actuated simultaneously and independently, there is a transition point between the theta motion path and the z motion path that is characterized by a sharp and sudden change in the valve's flow throttling capability (valve conductance), and the transition also typically includes an undesirable non-controllable flat regime through which active flow throttling cannot be effected. This non-linear valve conductance is shown in FIG. 9 and depicts the relationship between the pendulum valve plate position and the resultant vacuum chamber pressure. From this, it is evident that effective vacuum chamber pressure control relies heavily on three factors. They are:                a) the ability to move the valve plate quickly across the theta range of the valve stroke, since chamber pressure is highly insensitive to valve position in the theta valve stroke range;        b) the ability to produce active control along the z-axis, since this is where the majority of the controllable conductance can be realized. In addition, control along this axis has to be highly precise since chamber pressure is highly sensitive to valve position in the z-axis valve stroke range; and        c) the ability to reduce or eliminate the non-controllable flat regime near the transition between the theta- and z-motion respective stroke ranges.The inability of conventional valve geometry and actuation design to independently and simultaneously control theta and z-axis motion creates a severe limitation on the dynamic control range of the valve and/or linearity of control across the addressable stroke of the valve.        
With the conventional pendulum valve actuation and geometry, then, the user must accept nonlinear control characteristics and/or limited dynamic control range (especially when near-sealed in the z-axis direction) common with these types of valves that transition where the theta motion sequences to the z-axis motion and eventually seals.
What is needed is an improved pendulum valve and actuator mechanism and methodology that combines the high open conductance characteristics of a convention pendulum valve with improvements in its throttling capability garnered by simultaneous yet independent theta path and z-axis motion control. Further improvements should include a light and nimble plate design allowing for the fastest possible theta path motion, as well as valve plate and body design leading to maximum possible stroke length in the z-axis direction (and the ability to throttle there within).
Furthermore, it is desirable that the z-axis stroke (plate-to-flange travel distance when theta is in the closed or fully eclipsed position) be sufficiently large that conductance is uniform around the plate and that the majority of the control range be in the z-axis; since theta-path control yields non-uniform flow through the throat of the valve housing. Also, control in the z-axis direction produces less vibration than controlling in the theta-path direction because the moment of inertia about the z axis (torque=inertia*angular acceleration) is substantially greater than inertia created in the z axis (force=mass*linear acceleration).